Electrodynamic transducer



Jan. 5, 1965 G. c. TlBBETTS ELECTRODYNAMIC TRANSDUCER 2 Sheets-Sheet 1-Filed Sept. 21, 1959 zzz j'w was United States Patent Ofitice 3,164,686-Patented Jan. 5, 1965.

3,164,686 ELECTRQDYNAMIC TRANSDUCER George C. Tibbetts, Camden, Maine,assignor to Tibbetts Industries, Inc., Camden, Maine, a corporation ofMaine Filed Sept. 21, 1959, Ser. No. 841,280 Claims. (Cl. 179115.5)

One of the greatest problems in the high fidelity reproduction of soundis the occurrence of mechanical resonances within the various devicesused to make conversions between electrical and mechanical energy, soundwaves being a form of mechanical energy. These resonances occur when astructure of appreciable rigidity, mass, and extent is vibrated by alocally applied force. In the case of cone-type direct radiatorloudspeakers the need for an appreciable mass is occasioned by the rigidstructure needed to apply the motions of a comparatively small voicecoil to the comparatively large area from which acoustic radiation musttake place if appreciable acoustic power is to be delivered at the lowerfrequencies.

One attempt to alleviate these problems has been the development ofelectrostatic loudspeakers utilizing a diaphragm of very low mass.However, these speakers have generally been limited to the highfrequency ranges because the diaphragm excursion of such a speaker islimited unless the polarizing voltage is raised to the neighborhood ofseveral kilovolts and, for smaller excursions, extremely largediaphragms would be required to reproduce low frequency sound waves.

A further disadvantage of the electrostatic type loudspeakers is thatthey present a capacitive and high impedance load to the power sourcewhich is very difficult to match.

Accordingly objects of the present invention are to provide a transducerwith a wide and uniform frequency response which utilizes adiaphragm ofvery low rigidity and relatively low mass, and further to provide atransducer inherently capable of very low nonlinear distortion.

Further objects of the present invention are to provide such atransducer which is of simple and very reliable construction notrequiring close or precision gaps or other spacings, which does notrequire a polarizing voltage supply, and which lends itself to simpleand inexpensive mass production.

The present invention contemplates utilizing a generally planardiaphragm of very low rigidity which has an electric circuit coveringmost or all of its area. This circuit is disposed in a magnetic fieldwhich is edgewise the diaphragm and which islocally oriented so as toproduce aiding forces over the entire diaphragm as'a current is passedthrough said circuit.

The advantage of this general form of construction is that the drivingforce is applied to the diaphragm over a large part of its area just asis the air load. Consequently the diaphragm is not required to possessany great structural rigidity with its attendant parasitic mass.

In a preferred embodiment the circuit is constituted by metal foillaminated to a thin plastic film which constitutes the diaphragm. Apreferred method of construction is to laminate the metal foil to theplastic film in solid sheets and then to remove the unwanted metal bypreferential etching leaving the desired circuit.

'In order to make the air gap in the magnetic circuit as small aspossible it is desirable that the conducting path be narrow incomparison with theover-all dimensions of the diaphragm. Therefore, itis proposed that the path reverse its direction periodically so as tostill cover most or all of the area of the diaphragm. Accordingly, themagnetic field must reverse its direction with the same spatialperiodicity so that the forces produced by the instantaneous currentflow will be aiding over the entire current path, that is, so that themagnetic field will cross the current path in the same sense throughoutits length.

In one aspect the diaphragm is essentially rectangular and theconduction path covers it in parallel strips, each strip being joined atits one end to one adjacent strip and at its other end to the otheradjacent strip, adjacent strips being spaced from one another by anarrow gap. Correspondingly, magnetic poles may lie along the gapbetween adjacent strips-and in relatively close relationship to thediaphragm. Adjacent poles are then of opposite polarity so that thereexists a magnetic field, passing edgewise through the diaphragm andacross the current path, which reverses direction at every gap betweenadjacent conducting strips whereby a current through the circuit willproduce aiding forces over the entire diaphragm.

In a more specific aspect the poles may be those of bars havingessentially permanent dipole magnetization and lying along everyotherstrip in relatively close spaced relationship to the diaphragm, thebars being polarized edgewise of'the diaphragm and normal to theirlength, and each bar being magnetized similarly.

A more uniform magnetic field surrounding the conducting path may beobtained if a matching array of mag netic poles is placed on theopposite side of the diaphragm with poles of like polarity facing acrossthe diaphragm. In'this way the nonlinear distortion due to spatialvariation of the magnetic field as the diaphragm vibrates may beminimized. r

In a preferred embodiment the magnetic poles are the ends of magnetshaving their direction of polarization perpendicular to the plane of thediaphragm. The other ends of these magnets may rest against a structureof magnetically soft material so as to complete the magnetic circuitwith minimum reluctance.

In another embodiment the invention contemplates that the poles be theends of compact rectangular bar magnets lying with their direction ofpolarization parallel to the plane of the diaphragm whereby the oppositepoles lie along adjacent gaps between conducting strips, the magnets andcorrespondingly rectangular air spaces being arranged in the form ofacheckerboard array. In this case, magnets in one row, the row directionbeing normal to the direction of polarization, are magnetized oppositelyfrom those in an adjacent row.

When this device is used to produce acoustic energy, it is obvious thatthe diaphragm will radiate from both of itssurfaces. Especially at lowfrequencies where the wave length is large as compared with thedimensions of the transducer, the soundwaves may travel around the edgeof the diaphragm and cause destructive interference. In

, order to avoid this effect it is contemplated that thisdevice be usedwith a suitable enclosure for isolating the back radiation or fordelaying it so that it appears in phase with the forward radiation. Suchenclosures are well understood in the art and form no part of thepresent invention.

It is also contemplated that combinations of units ac-' cording to thepresent invention may be assembled in; order to handle more power, or inorder to cover a greater range of frequency or, for a given range offrequencies, to minimize Doppler type frequency modulation distortion. 1In the latter case the units may diifer in detail such as size anddiaphragm construction so as to make each suited to reproduce aparticular band of frequencies. Accordingly the electrical energydelivered to the com- I bination may be divided according to frequencybefore a going to the transducers.

Such cross-over networks are also well understood in the art and form nopart of the present invention.

In the alternative, a single diaphragm may be divided into electricallyisolated regions with each region being driven by separate sources suchas frequency dividing anegese networks or amplifiers. In this way therequirements as to driving area and mass considerations may becontrolled so as to be suitable for various frequency ranges withoutrequiring more than one diaphragm and mounting structure.

For the purpose of illustration several embodiments of the presentinvention are shown in the accompanying drawings wherein:

FIG. 1 is a partially broken away isometric view of a transducer;

FIG. 2 is a section substantially on the line 22 of FIG. 1;

FIG. 3 is a section substantially on the line 33 of FIG. 2; FIG. 4 is aplan view of a modification;

FIG. 5 is a section on the line 5-5 of FIG. 4;

FIG. 6 is a plan view of a further modification;

FIG. 7 is a section on the line 77 of FIG. 6;

FIG. 8 is a plan view of a diaphragm;

FIG. 9 is a plan view of a modification of the diaphragm;

FIG. 10 is a view on the line 1010 of FIG. 9 also showing the positionof the magnets; and

FIG. 11 is a plan view of a further diaphragm modification.

Referring now to FIGS. 1, 2 and 3, the diaphragm 1 is constructed of aplastic film such as .0005 inch thick Mylar and is clamped at its edgesas a membrane by gaskets 2. Laminated to the diaphragm 1 is a foil 3 ofan electrically conducting material such as .001 inch thick aluminum.The foil 3 is disposed in parallel strips 4 each of which is connectedat one end to one of the adjoining strips and at the other end to theother adjoining strip thereby defining a zig-zag pattern covering mostof the free surface of the diaphragm 1. Disposed adjacent the gaps 5between adjoining strips 4 are elongate magnetic pole pieces 7. Thesepole pieces 7 derive their polarity from elongate permanent magnets 8whose direction of polarization is perpendicular to the plane of thediaphragm. The magnets are arranged so that adjoining pole pieces 7 areof opposite polarity, and so that pole pieces 7 of like polarity areopposed across the diaphragm 1. A perforate backing plate 9 of amagnetically soft material such as iron supports the magnets 8 andcompletes the magnetic circuit while permitting the free passage ofsound waves through the apertures 11, the structure is substantiallyacoustically transparent.

Preferably the pole pieces 7, the magnets 8, and the backing plate 9 incombination are shaped so as to form an aerodynamically smoothstructure. The pole pieces 7 may be omitted and replaced by extensionsof the magnets 8.

In the embodiment shown the backing plate 9 comprises as an integralstructure, a grid of intersecting bars 17 and 18. The bars 17 coveringthe magnets 8 may be of generally semi-elliptical cross-section so as toform a smooth cross-section with the magnets 8 and the pole pieces 7.The transverse bars 18 may be, of themselves, of ellipticalcross-section.

While the backing plate is desirable for purposes of increasing themagnetic efficiency, it is obvious to one skilled in the art that theplate might be omitted and the magnets made self-supporting.

In order to minimize nonlinear distortion from mechanical causes duringa substantial vibration excursion, the diaphragm 1 and the foil 3 may bebulged locally into dimples 12 which by increasing the effectivethickness of the diaphragm, without appreciably increasing the mass,increase the range of quasi-linear deflection of the diaphragm compositeas a flexural device. As shown by the broken lines in FIG. 1, adjacentdimples 12 may be directed in opposite directions.

On the sides parallel to the magnets 8 the frame 13 is constituted by anextension of the backing plate 9. This extension 14 is spaced from theend magnet 8 so that lit lies along a magnetic equipotential, that is ata distance equal to half the spacing between adjacent magnets 8.

On the sides transverse the magnets 8 the frame 13 is constituted by asidewall 15 of a non-ferromagnetic material so as to cause minimumdistortion of the magnetic field.

While in the embodiments shown there are magnetic structures on bothsides of the diaphragm in order to produce a more uniform magnetic fieldover the diaphragms excursion, it is to be understood that a lessexpensive embodiment workable especially at the higher frequencies maybe obtained by completely omitting the magnetic structure on one side.

In the embodiment depicted in FIGS. 4 and 5 the elongate magnets 20 havebeen oriented with the direction of polarization parallel to thediaphragm 21 and perpendicular to the strips 22 of conducting material.The pole faces of the magnets 20 lie adjacent the gaps betweenconducting strips 22 and the magnets are spaced by a distance equal totheir width, whereby the magnetic field configuration resembles that inthe embodiment depicted in FIGS. 1, 2 and 3 and there is approximatelyfifty percent open area, rendering the structure substantiallyacoustically transparent.

In the embodiment depicted in FIGS. 4 and 5 the magnetic field isterminated without appreciable distortion, that is, approximately as ifthe array extended indefinitely, by including magnets 23 of half widthand a magnetically permeable side plate 24 having its inside surfacelocated at the magnetic equipotential.

In this embodiment it is apparent that the diaphragm 21 cannot be bulgedin alternately opposite senses along both dimensions Without increasingthe danger that it will strike a magnet when vibrating. However, the diaphragm 21 may be dimpled in a quilted pattern in the same sense in anyrow along a magnet 20 and in the opposite sense in an adjacent row, thediaphragm always bulging away from the nearest magnet.

In the embodiment shown in FIGS. 6 and 7 the magnets 30 are arranged ina checker-board pattern, again with the direction of polarizationparallel the diaphragm 33 and with the poles of the magnets 30 lyingadjacent the gaps between conducting strips 31. The magnets in each roware spaced by distance equal to their width and each abuts a spacebetween magnets in the adjacent row, whereby there is approximatelyfifty percent open area, rendering the structure substantiallyacoustically transparent. The magnets 30 are supported by bars 32parallel to the rows which preferably are of a nonferro' magneticmaterial.

If magnets 30 are to be placed on both sides of the diaphragm 33, thetwo arrays may be arranged so that each magnet 30 is opposite an opening35 between magnets in the opposing array.

Again the field may be effectively terminated by the inclusion ofmagnets 36 of half width and a magnetically permeable side wall 37 atthe magnetic equipotential.

In this embodiment, as in the first, the diaphragm may be locallystiffened by bulging or dirnpling if there is one dimple 33 for eachmagnet 30 and that dimple bulges away from the magnet.

FIG. 8 shows a diaphragm constructed as a laminate of plastic film andmetallic foil. Such a diaphragm may be constructed by laminatingcontinuous sheets of plastic film and foil and then creating the gaps 43by preferential etching. The edge strips 45 of foil may be left tostrengthen that part of the diaphragm although it will conduct nocurrent. Means for connection 46 are provided to connect the zigzag foilconductor to an external circuit.

A similar diaphragm might also be constructed by other printed circuittechniques or by other means of metal deposition.

1 In the modification of the diaphragm depicted in FIGS.

and 11 (shown flat for purposes of simplicity) there are conductingstrips 51 on both sides of the plastic film 50. Each strip is spacedfrom the adjoining strips on the same side of the film 50 by a distanceslightly less than the width of a strip and the strips on opposite sidesare staggered so that they overlap slightly. In this way all portions ofthe diaphragm are supported by foil without appreciably changing themass of the diaphragm. The width of these strips will be approximatelyhalf the distance between poles 52 so that an aiding current-fieldrelationship may be maintained. As each of the embodiments utilizes anessentially similar magnetic field configuration, this diaphragm may beused with any embodiment shown. A primary advantage of this form ofdiaphragm is that an essentially constant current density can bemaintained out to the equipotential terminating surfaces 'of themagnetic structure, as shown by dotted lines in FIGS. 9 and 10, andhence that the diaphragm is driven by distributed electrodynamic forcesover the entire area provided with a substantial magnetic field, withoutwaste of this area.

A further advantage is that, for a given thickness of foil and with thetwo sides connected in series, the input impedance of the device will benearly quadrupled which will generally aid in coupling of the device toan external circuit.

FIG. 11 shows a single diaphragm divided by gaps 53 into electricallyisolated regions 54, 55 and 56. Edge strips 57 of foil may be left forstrength. Means for connection 58 are provided to connect each region torespective external circuits, in the form of separate driving sourcessuch as frequency dividing networks or amplifiers.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

I claim:

1. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, a thin flexible conductor adhering to saiddiaphragm for conducting current edgewise the diaphragm along apredetermined path, and magnetic means including magnetic poles mountedon said support adjacent the diaphragm with said poles lying in a planespaced from and parallel to the diaphragm for producing a magnetic fieldedgewise the diaphragm which extends across said path always in the samedirection, relative the path throughout the length of the path, theconducting path reversing direction periodically, the conducting pathbeing arranged in two series of parallel bands, one series on each sideof the diaphragm, each band being spaced from the adjoining bands on thesame side of the diaphragm by a distance slightly less than the width ofa band and each band being connected at one end to one of said adjoiningbands and at the other end to the other of said adjoining bands, andbands on the opposite sides of the diaphragm being staggered so thatthere is a slight overlap.

2. An electrodynamic transducer comprising a membranous diaphragm havingconducting means disposed upon the surface thereof, said conductingmeans being arranged in two series of parallel bands, one series on eachside of the diaphragm, each band being spaced from the adjoining bandson the same side of the diaphragm by a distance slightly less than itswidth and connected at one end to one of said adjoining bands and at theother end to the other adjoining band, bands on the opposite sides ofthe diaphragm being staggered with a slight overlap, and a plurality ofmagnetic pole pieces lying in a plane parallel and closely adjacent theplane of the diaphragm, each pole piece lying adjacent the divisionbetween bands carrying current in opposite directions and adjacent polesbeing of opposite polarity.

3. An electrodynamic transducer comprising a diaphragm having disposedthereupon an electrical circuit,

said circuit being arranged so as to form a series of parallel lines inwhich each line is joined at one end to one of the adjacent lines and isjoined at the other end to the other adjacent line, a suspension forsaid diaphragm, a plurality of permanent magnets lying in a planeparallel and closely adjacent the plane of the diaphragm, said magnetsbeing arranged in parallel rows with all of the magnets in one row beingoriented alike transverse the length of the row and spaced from oneanother by a distance substantially equal to their width, the poles ofsaid magnets being located adjacent the spaces between adjacent lines onthe diaphragm, the magnets in adjacent rows being oppositely orientedand staggered so that each pole adjoins a space between magnets in theadjoining row.

4. An electrodynamic transducer accordingto claim 3 wherein there is asecond plurality of magnets generally similar to the first but locatedon the other side of the diaphragm and disposed so that each magnet isopposite a space between magnets in the opposing plurality of magnetsand so that like poles always lie opposed across the diaphragm.

5. A transducer according to claim 4 wherein the diaphragm is bulgedlocally so as to linearize flexurally the diaphragm, said bulges alwaysextending in a direction away from any magnet.

6. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, the diaphragm being linearized flexurally bydimples bulged up from the substance of the diaphragm, a thin flexibleconductor adhering to said diaphragm for conducting current edgewise thediaphragm along a predetermined path, and magnetic means includingmagnetic poles mounted on said support adjacent the diaphragm with saidpoles lying in a plane spaced from and parallel to the diaphragm forproducing a magnetic field edgewise the diaphragm which extends acrosssaid path always in the same direction relative the path throughout thelength of the path.

7. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, the diaphragm being linearized flexurally bydimples bulged up from the substance of the diaphragm in a quiltedpattern, a thin flexible conductor adhering to said diaphragm forconducting current edgewise the diaphragm along a predetermined path,and magnetic means including magnetic poles mounted on said supportadjacent the diaphragm with said poles lying in a plane spaced from andparallel to the diaphragm for producing a magnetic field edgewise thediaphragm which extends across said path always in the same directionrelative the path throughout the length of the path.

8. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, a thin flexible conductor adhering to saiddiaphragm for conducting current edgewise the diaphragm along apredetermined path, said conducting path reversing directionperiodically in space, and magnetic means including magnetic polesmounted on said support adjacent the diaphragm with said poles lying ina plane spaced from and parallel to the diaphragm for producing aperiodically reversing magnetic field edgewise the diaphragm such thatthe magnetic flux extends across said path always in the same directionrelative the path throughout the length of the path, the diaphragm beingdivided into electrically isolated regions, the conducting path withineach region including means for connection with an external circuit.

9. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, a thin flexible conductor adhering to saiddiaphragm for conducting current edgewise the diaphragm along apredetermined path, and substantially acoustically transparent, magneticmeans including magnetic poles mounted on said support adjacent thediaphragm with said poles lying in a plane spaced from and parallel tothe diaphragm for producing 7 a magnetic field edgewise the diaphragmwhich extends across said path always in the same direction relative thepath throughout the length of the path, the conducting path reversingitself periodically and being arranged in two series of parallel bands,one series on each side of the diaphragm, wherein each band is spacedfrom the adjoining bands on the same side of the diaphragm by a distanceslightly less than the Width of a band and each band is connected at oneend to one of said adjoining bands and at the other end to the other ofsaid adjoining bands, and wherein bands on the opposite sides of thediaphragm are staggered so that there is a slight overlap.

10. A transducer comprising a support, a flexible diaphragm held at itsedges by the support, a thin flexible conductor adhering to saiddiaphragm for conducting current edgewise the diaphragm along apredetermined path, said conducting path reversing directionperiodically in space, and substantially acoustically transparentmagnetic means including magnetic poles mounted on said support adjacentthe diaphragm with said poles lying in a plane spaced from and parallelto the diaphragmfor producing a periodically reversing magnetic fieldedgewise the diaphragm such that the magnetic flux extends across saidpath always in the same direction relative the path throughout thelength of the path, the magnetic field being terminated at a magneticequipotential by a magnetically permeable member at the edges of thediaphragm paralleling the conducting path.

- References Cited by the Examiner UNITED STATES PATENTS 484,339 7/24Lorenz et al. 179.115.5 1,537,671 5/25 Hewlett 179115.5 1,601,656 9/26Thayer 179181 1,604,532 10/26 Riegger 179115.5 1,676,668 7/28 Round1'79--181 1,749,635 3/30 Gerlach 179181 1,793,483 2/31 Hewitt 1791811,955,390 4/34 Schifii 179115.5 2,003,908 6/35 Smith 179-181 2,041,1635/36 Burke 179l81 2,044,608 6/36 Harford 179181 2,293,372 8/42 Vasilach179115.5 2,404,798 7/ 46 Harry et al. 179115.5 2,535,757 12/50 Root179-115 2,978,671 4/61 Harris 179-1155 3,013,905 12/61 Gamzon et al.179--115.5

FOREIGN PATENTS 227,162 1/ 25 Great Britain.

329,328 5/30 Great Britain.

484,339 7/ 24 Germany.

OTHER REFERENCES Publication: Elements of Loudspeaker Practice, by N. W.McLachlan, London, Oxford University Press, 1935.

ROBERT H. ROSE, Primary Examiner.

L. MILLER ANDRUS, WILLIAM C. COOPER,

Examiners.

1. A TRANSDUCER COMPRISING A SUPPORT, A FLEXIBLE DIAPHRAGM HELD AT ITSEDGES BY THE SUPPORT, A THIN FLEXIBLE CONDUCTOR ADHERING TO SAIDDIAPHRAGM FOR CONDUCTING CURRENT EDGEWISE THE DIAPHRAGM ALONG APREDETERMINED PATH, AND MAGNETIC MEANS INCLUDING MAGNETIC POLES MOUNTEDON SAID SUPPORT ADJACENT THE DIAPHRAGM WITH SAID POLES LYING IN A PLANESPACED FROM AND PARALLEL TO THE DIAPHRAGM FOR PRODUCING A MAGNETIC FIELDEDGEWISE THE DIAPHRAGM WHICH EXTENDS ACROSS SAID PATH ALWAYS IN THE SAMEDIRECTION, RELATIVE THE PATH THROUGHOUT THE LENGTH OF THE PATH, THECONDUCTING PATH REVERSING DIRECTION PERIODICALLY, THE CONDUCTING PATHBEING ARRANGED IN TWO SERIES OF PARALLEL BANDS, ONE SERIES ON EACH SIDEOF THE DIAPHRAGM, EACH BAND BEING SPACED FROM THE ADJOINING BANDS ON THESAME SIDE OF THE DIAPHRAGM BY A DISTANCE SLIGHTLY LESS THAN THE WIDTH OFA BAND AND EACH BAND BEING CONNECTED AT ONE END TO ONE OF SAID ADJOININGBANDS AND AT THE OTHER END TO THE OTHER OF SAID ADJOINING BANDS, ANDBANDS ON THE OPPOSITE SIDES OF THE DIAPHRAGM BEING STAGGERED SO THATTHERE IS A SLIGHT OVERLAP.
 7. A TRANSDUCER COMPRISING A SUPPORT, AFLEXIBLE DIAPHRAGM HELD AT ITS EDGES BY THE SUPPORT, THE DIAPHRAGM BEINGLINEARIZED FLEXURALLY BY DIMPLES BULGED UP FROM THE SUBSTANCE OF THEDIAPHRAGM IN A QUILTED PATTERN, A THIN FLEXIBLE CONDUCTOR ADHERING TOSAID DIAPHRAGM FOR CONDUCTING CURRENT EDGEWISE THE DIAPHRAGM ALONG APREDETERMINED PATH, AND MAGNETIC MEANS INCLUDING MAGNETIC POLES MOUNTEDON SAID SUPPORT ADJACENT THE DIAPHRAGM WITH SAID POLES LYING IN A PLANESPACED FROM AND PARALLEL TO THE DIAPHRAGM FOR PRODUCING A MAGNETIC FIELDEDGEWISE THE DIAPHGRAM WHICH EXTENDS ACROSS SAID PATH ALWAYS IN THE SAMEDIRECTION RELATIVE THE PATH THROUGHOUT THE LENGTH OF THE PATH.